Solar selective coatings having high solar absorptance in the solar spectrum and low thermal emittance at elevated temperatures with enhanced thermal stability (>400° C.) are needed on solar collector receiver tubes to increase the overall solar to electric efficiency of parabolic trough solar power plants [C. Kennedy and H. Price, Proceedings of ISEC2005, 2005, International Solar Energy Conference, August 6-12, Orlando, Fla., USA]. Increasing the operating temperature of the receiver tube reduces the cost of parabolic trough technology and hence the cost of solar electricity.
A large number of high-temperature solar selective coatings have been developed in the recent past for high temperature applications using a combination of high temperature stable materials. Cermets based on Mo—SiO2[S. Esposito et al., Thin Solid Films 517 (2009) 6000], W—Al2O3 [A. Berghaus et al., Solar Energy Materials & Solar Cells 54 (1998) 19, A. Antonaia, Solar Energy Materials & Solar Cells 94 (2010) 1604], Mo—Al2O3 [D. Xinkang et al., Thin Solid Films 516 (2008) 3971] and multilayer absorber consisting compounds of Ti, Al, Si, etc., [H. Lei et al., Chinese Science Bulletin 54 (2009) 1451, Du M. et al., Solar Energy Materials & Solar Cells 95 (2011) 1193].
Reference may be made to applicant's U.S. Pat. No. 7,585,568 and also to international patent application WO 2009/051595 A1, wherein efforts have been made to improve the thermal stability of the absorber coatings. Coatings disclosed therein are thermally stable at approximately 450° C. However, even today there is great challenge to develop solar selective coatings that have high solar absorptance and low thermal emittance for temperatures greater than 400° C. in vacuum/air.
Earlier, the applicant had developed high temperature solar selective coatings for solar thermal power generation and patent applications (3655 DEL 2011, PCT/IN 2012/000451 and 0371DEL2013) are under process. Patent application 3655 DEL 2011 discloses a multilayer solar absorber coating containing tandem stacks of Ti/chrome interlayer, AlTiN, AlTiON and ATiO deposited on SS 304 and other substrates, and exhibits absorptance of 0.930 and emittance of 0.16-0.17 with a thermal stability of 450° C. in vacuum. The chrome layer in this invention was deposited using electrodeposition, whereas the other absorber layers were deposited using sputtering technique. In the second invention (Indian patent application 0371DEL2013 & PCT/IN 2012/000451) the applicant had developed an improved hybrid solar absorber coating using sputtering and sol-gel techniques. This hybrid multilayer absorber coating exhibits absorptance of 0.950, emittance of 0.11 and thermal stability up to 600° C. in vacuum. The above referenced inventions of the applicant had two major limitations: (1) The lowest emittance was 0.11 on stainless steel substrates and (2) the inventions used more than one deposition process (for example, a combination of sputtering, electrodeposition and sol-gel). These limitations of the earlier inventions inspired the inventors to develop an improved high temperature solar selective coating with very low thermal emittance using a sputtering process.
Recently published references related to the improvement in solar selective coatings, are discussed below.
References may be made to B. Weber et al., Structure and Reactivity of Surfaces, Editors C. Morterra et al., Elsevier Science Publishers, B.V. Amsterdam page 919 (1989), wherein optical emissivity during the first stage of the oxidation of W has been studied. A significant increase in the emissivity of W in the temperature range 800-1000 K has been observed because of the formation of WO3.
References may also be made to A. Antonaia, Solar Energy Materials & Solar Cells 94 (2010) 1604, wherein W—Al2O3cermet based solar coatings for receiver tube applications have been developed. These coatings display solar absorptance of 0.93 and hemi-spherical emittance of 0.14 at 550° C. and were found to be stable up to 30 days after annealing at 580° C.
References may be made to S. Esposito et al., Thin Solid Films 517 (2009) 6000, wherein Mo/Mo—SiO2 multilayer graded cermet coating has been developed for high-temperature applications. The graded multilayer cermet coating shows solar absorptance of 0.94 and low thermal emittance at high temperature (<0.13 at 580° C.).
References may be made to Du M. et al., Solar Energy Materials & Solar Cells 95 (2011) 1193, wherein Ti0.5Al0.5N/Ti0.25Al0.75N/AlN coatings on copper coated silicon substrates have been developed for solar selective applications. The optimized coatings exhibited absorptance of 0.945 and emittance of 0.04 at 82° C. However, no thermal aging tests have been carried out in this paper.
References may also be made to H. Lei et al., Chinese Science Bulletin 54 (2009) 1451, wherein TiAl/TiAlN/TiAlON/TiAlO coating was deposited on SS and copper substrates using a multi-arc ion plating system. The coating exhibits absorptance of 0.90 and emittance of 0.09-0.19 and is found to be stable in air up to 650° C. for 1 hour. This process uses a multi-arc ion plating system in which it is extremely difficult to control the thicknesses of component layers on a nanometric scale. Also, long term thermal stability tests have not been reported in this paper.
References may be made to C. Sella et al., Solar Energy Materials 16 (1987) 143, wherein low cost selective absorber based on Fe—Al2O3 cermet film has been deposited on stainless steel substrates. The steel substrate is covered with an infrared reflector consisting of Mo or W. The absorber coating exhibits absorptance of 0.950 and emittance of 0.06. The long term thermal stability tests have not been carried out by these authors. The Fe present in the absorber coating is highly unstable as it forms iron oxide when exposed to air even at room temperature and is not an ideal coating for high temperature applications.
Reference may also be made to U.S. Publication No. US 2011/0249326A1, wherein solar absorber coating consisting of metal, dielectric or ceramic material along with a highly reflective metal layer is manufactured on metallic substrates. For the reflective layer, silver, gold, titanium, chromium, molybdenum, copper, nickel, titanium, niobium, tantalum, tungsten and palladium are used. The absorber layer consists of a multilayer structure consisting of metal and dielectric. The dielectrics are selected from metal oxides and metal element nitrides. The substrate is coated with a Mo layer of 300 nm thickness. On this layer of Mo a multilayer structure consisting of two zones is deposited. The first zone consists of 285 layers of SiAlOx each having a thickness of 0.08 nm alternated with 285 layers of Mo with a thickness of 0.1 nm. The second zone consists of 390 layers of SiAlOx with a thickness of 0.08 nm alternated with another 390 layers of Mo with a thickness of 0.06. Over these layers an anti-reflection layer of SiAlOx has been deposited with a thickness of 87 nm. The coating shows absorptance of 97.5% and emissivity of 15% at 400° C. This invention uses metal layers of thicknesses less than 1 nm for the absorber layer, which may get oxidized when exposed to air. Also, depositing as many as 675 layers each of metal and dielectric makes the process cumbersome. Another disadvantage of this invention is that the emittance is 0.15 at 400° C.
To manufacture an efficient solar absorber coating for solar thermal power generation applications, its absorptance should be as high as possible (>0.950) and the emittance should be as low as possible (<0.10) on thermally stable metallic substrates such as stainless steel. Low emittance of the absorber coating is very much desirable as the radiative losses are proportional to T4, where T is the operating temperature of the receiver tube applications (generally >400° C.). For receiver tubes generally stainless steel is used, which has very high intrinsic emittance (of the order of 0.11-0.12). Therefore, a suitable infrared reflecting layer with improved thermal stability is required for designing a solar selective coating with high spectral selectivity on stainless steel substrates. Additionally, use of the single process (such as sputtering) is necessary for efficient manufacturing of the receiver tubes.
As seen from the prior-art, reducing the thermal emittance of absorber coating on stainless steel alone is a major requirement of efficient solar selective coating. A desired substrate is stainless steel because of its high thermal stability, high corrosion resistance and ease of manufacturability. It is also desired to have high temperature compositionally stabile absorber coatings using a facile manufacturing process and these issues are still relevant even today. Therefore, development of an efficient high-temperature absorber coating using a single process is needed for solar thermal power generation.
W as the infrared reflector, in embodiments of the present invention, is deposited first on a stainless steel substrate using a sputtering process. The W interlayer helps in reducing the overall emissivity of the multilayer absorber coating. Additionally, the other constituent layers of the present invention have been chosen in such a way that the absorber coating exhibits absorptance>0.950, emittance<0.08(at 80° C.) on stainless steel substrates and thermal stability in vacuum up to 600° C. under cyclic heating conditions.
In embodiments of the present invention, the infrared emittance of stainless steel was controlled using a W coating having thickness between 830-1000 nm deposited by a sputtering using a balanced magnetron sputtering system and all the other constituent layers of the tandem stack have been deposited using a four cathode reactive unbalanced pulsed direct current magnetron sputtering technique. The first tungsten layer deposited on substrate decreases the thermal emittance of stainless steel 304 to 0.03-0.04 from its intrinsic emittance of 0.12-0.13. The W coating reduces the emittance of SS 304 to 0.03-0.04 from its intrinsic emittance of 0.11-0.12. The W coated SS 304 substrate was used to deposit high temperature solar selective coating consisting a tandem stack of titanium aluminum nitride, titanium aluminum silicon nitride, titanium aluminum silicon oxy-nitride and titanium aluminum silicon oxide deposited using a four cathode reactive pulsed direct current magnetron sputtering system. The present invention, thus, provides a multilayer solar selective coating having absorptance>0.950, emittance≦0.08 (on SS 304) and high thermal stability in vacuum at 600° C. up to 1000 hours under cyclic heating conditions. The solar selective coating of the present invention is stable up to 350° C. in air for more than 1000 hours under cyclic heating conditions and also displays thermal shock resistance up to 375° C. for more than 100 cycles. The solar selective coating of the present invention displays improved optical properties and thermal stability suitable for high temperature solar thermal power generation